C# 將短期格式變長正常時間格式

               DateTime dateTimeStart = TimeZone.CurrentTimeZone.ToLocalTime(new DateTime(1970, 1, 1));
                long lTime = long.Parse(time_end + "0000000");
                TimeSpan toNow = new TimeSpan(lTime);
                DateTime notifyTime = dateTimeStart.Add(toNow);